Modern microelectronic devices are commonly produced using a photolithographic process. In this process, a semiconductor wafer is first coated with a layer of photoresist. This photoresist layer is then exposed (during a so-called exposure process) to illuminating light using a reticle (also called photo-mask, or a mask) and subsequently developed. Upon development, exposed positive photoresist is removed (alternatively, non-exposed negative photoresist is removed), and the remaining photoresist produces the image of the reticle on the wafer. Thereafter, the uppermost layer of the wafer is etched. Thereafter, the remaining photoresist is stripped. For multilayer wafers, the above procedure is then repeated to produce subsequent patterned layers.
It should be appreciated by those skilled in the art that to produce an operational semiconductor product, a reticle must be as defect-free as possible, preferably completely defect-free. Therefore, reticle evaluation tools are needed to detect various defects in the reticles that can potentially reduce the microelectronic circuit fabrication yields.
It is customary to relate the electric field just behind the mask (reticle) to the field of impinging light by a mask function. The notion of a mask function can be useful also for imaging and projection applying partially coherent light, where in that case the image formation is typically described by some variant of the Hopkins formula. For a mask with pattern features large compared with the illuminating wavelength, the Kirchoff boundary conditions, also known as the “thin mask approximation”, can be safely used in order to compute the resulting aerial image or, alternatively, the intensity pattern on the wafer. However, once the features on the mask become comparable in size to the illuminating wavelength, as is the case with state-of-the-art current masks, this approximation is no longer valid, and the transmission of the mask becomes dependent on the polarization state and angle of incoming light.
For certain features which are common in modern reticles, typically ones possessing a high degree of spatial symmetry, the amplitude, phase and polarization state of diffracted light can be predicted, based, for example, on analytic results as well as efficient numerical models. For other reticle patterns, where such calculations are either very inefficient or too complicated, one may still use the knowledge of the reticle design, together with certain interferometric or wavefront-sensing technique, in order to obtain a sufficiently close approximation of the properties of the diffraction orders that carry most of the image power.
Rapid shrinking of feature size in semiconductor products has led to an increasing Numerical Aperture (NA) values in exposure systems such as steppers. Currently, the Numerical Apertures of exposure systems exceed one, are growing rapidly towards one point four and are expected to reach one point eight in the near future.
At high Numerical Aperture values which represent large angles of incidence, the exposure process is more susceptible to polarization related effects, especially those effects that occur at high angles of incidence of the light upon the resist.
Aerial imaging tools, such as aerial inspection tools and aerial review tools, try to mimic the exposure process while applying an imaging process that differs from the exposure process. While during the exposure process the image of the reticle is de-magnified, during the imaging process the image of the reticle is magnified. This magnification results in a decrement of the angles of incidence by the same magnification factor, which can be around several hundreds. The consequence of this magnification process is that polarization effects that occur on the wafer plane are not emulated by the aerial imaging system. Especially, an exposure process substantially reduces the contrast ratio of p-polarized light (also referred to as TM) in relation to the contrast ratio of s-polarized light (also referred to as TE), while an imaging process does not perform such a reduction.
FIGS. 1 and 2 illustrate the difference between p-polarized light components and s-polarized light components.
FIG. 1 illustrates an exposure system 10 that includes light source 12 that provides s-polarized light. The diffraction creates two or more coherent rays (such as rays 15a and 15b of FIG. 1), that appear at different locations on the pupil plane, each arriving at the image plane from a different direction.
As the coherent rays hit the wafer from different directions, all their fields vectors combine in a vector superposition manner, to create the point electric field.
The aerial image is the intensity of this field. The angular difference between the two incident beams creates the high-angle polarization effect.
Light source 12 is followed by reticle 14 and by objective lens 16. S-polarized light passes through transparent portions of reticle 14 towards objective lens 16 that projects an image of reticle 14 onto wafer 20. The electric field vector of the s-polarized light is perpendicular to the plane of FIG. 1. Extract 30 is a top view of the electric field vector of radiation of the s-polarized light at the pupil plane, or at an upper portion of objective lens 16, while extract 32 is a side view of the coherent light rays 42 and 44 at the wafer (or image) plane. Two dots 46 and 48 represent the direction of the field vectors of these light rays.
FIG. 2 illustrates the same configuration as an exposure system 10 but light source 12 provides p-polarized light. The intensity vector of the p-polarized light appears in the plane of FIG. 1. Extract 34 is a top view of the electric field vector of radiation of the p-polarized light at an upper portion of objective lens 16 while extract 36 is a side view of the coherent light rays 52 and 54 at the wafer plane. Two dots 56 and 58 represent the field vector associated with these light rays. It is noted that every illumination source may be described as generating radiation rays that include both S and P polarizations components.
The aerial image is the square self dot product of the electric field vector of light that impinges on a wafer or on a sensor. The electric field can be divided into two independent polarization components—p-polarized (TM) component and s-polarized (TE) component. The theory of imaging at high values of numerical aperture indicates that the vector sum of s-polarized light is markedly different from that of p-polarized light, due to the large angles of incidence encountered at high Numerical Aperture projection. Indeed, while the angle between the electric field vectors of two s-polarized beams that converge on the image plane from different directions is independent of the polar angles of the beams (the angles between the beams and the system's optical axis), this is certainly not the case for two p-polarized such beams. This implies a strong dependence of the contrast of images formed by p-polarized light on the angle of incidence at the image plane. As the typical feature size on the photo-mask shrinks, light converging at high angles of incidence carries the significant fraction of the power at the spatial frequencies required for successful image formation, whence the critical dependence on the beams at the extreme angles of incidence.
Moreover, at NA values exceeding ˜0.7, both regimes give rise to images whose properties may deviate significantly from those obtained with low numerical aperture imaging, owing to the enhancement of the relative power carried by converging beams of high angle of incidence.
While projection systems of an exposure process typically employ very high angles of incidence, the large magnification applied by the imaging process results in light that converges on the image plane with much smaller angles of incidence. This fundamental difference results in different images produced by the same mask pattern on the wafer and at the image plane of the exposure system and imaging system, respectively.
These differences between the exposure process and the imaging process should be compensated for in order to enable imaging systems to mimic the exposure process in a reliable manner.
There is a need to provide efficient systems and methods for reticle evaluation.